I have more experience with ASP since that is what out system is made in (crappy, I know, our last programmer was a moron). I am relitavely new to ASP.net, but I like it more than PHP for these reasons:
-Speed: Most of the time, the code is pre-compiled into MSIL code. So it runs at a far lower level than PHP so you gain speed.
-Power: You can use VB, C#, C++, and a number of other languages. I prefer c# since VB is a basic language and C++ is harder to work with (you have to compile every time). They are almost identical to being used outside of a web environment. The .net framework makes a lot of things easier.
-Code Separation: Using code behind (a built in feature with VS 2005+), you can separate the core logic from the code without using a templating engine.

Now, for what I don't like about it in comparison to PHP:
-Complication: PHP is a lot like C++ but far simpler. You are either stuck with a basic language, C#, or C++. C# and C++ being rather hard to work with. Visual Studio is also harder to use (but more capable than most IDE's). It can also be annoying compiling your DLLs to put on the web
-Learning curve: It is different than PHP, its hard to explain. It has a learning curve to get the hang of things. It did for me anyway and I pick up on most languages in a few minutes.

__________________